Any word ending in ون drops the final ن if it goes into إضافة - and that final و then declines depending on grammatical case
ذهب مديرو الشركة
قتل المجرم مديرا المخبز - I think this is right but people may (wrongly) perceive it to be in the dual
فرصة لمديري الموارد البشرية
هؤلاء مديرون أكفاء
I wouldn't be surprised to see مدراء as an alternative plural but I wouldn't use it myself.